Why is the C H bond in ethene (H2CCH2) shorter and stronger than the C H bond in ethane(H3C-CH3)?

The length and strength of a C H bond depends on the hybridization of the carbon atom. The more s character in the hybrid orbital used by carbon to form the bond, the shorter and stronger the bond. This is because an s orbital is closer to the nucleus than is a p orbital. Ethene uses carbon sp2 hybridized orbitals (1/3 s character) to make its carbon-hydrogen bonds while ethane uses carbon sp3 (1/4 s character) orbitals.